In clean rooms, such as bioclean rooms, airborne microbial particles and non-microbial particles are detected and recorded using particle detecting devices. From the result of particle detection, the degree of wear of air-conditioning equipment in the clean room can be ascertained. The record of particle detection performed in the clean room may be attached, as a reference, to products manufactured in the clean room. An optical-type particle detecting device, for example, draws in gas in the clean room and illuminates the drawn-in gas with excitation light. If the gas contains microbial particles or non-microbial fluorescent particles, the particles illuminated by the excitation light emit fluorescence. This allows detection of the numbers and sizes of microbial particles and non-microbial fluorescent particles contained in the gas (see, e.g., PTL 1). There are demands for techniques that allow accurate detection of particles contained in fluid, not only in clean rooms but also in other environments. Here, the term fluid includes liquids as well as gasses.